This invention is directed to a process for continuous control of cell number in the production of polyurethane foams from polyols, isocyanates and water, where mixing the components is effected in a mixing head with adjustable injection pressure and adjustable mixing chamber pressure.
From DE-A-24 40 022, a process and a device is known for the production of insulation and lightweight building materials from cold- or hot-setting reaction and/or condensation resins, where the resin is loaded with the required amount of propellant in a pressure container, and the foam is generated by depressurizing the mixtures. Here, loading of resin with propellant is accelerated by increasing the gas/liquid boundary surface, omitting moving stirring assemblies, whereby a foam is formed from the one component and the propellant by mere mechanical means. This preformed foam then is mixed with the second component. Final foam production then is effected by depressurizing and curing the mixture.
From DE-A-39 20 994, there is known an improved device for producing a reaction mixture from at least two components for the production of foams, where at least one of the feed lines has a gas feed device with a gas feed gauge and a volume flow counter. Here, the gas feed device consists of a special static mixer, and gas feed is controlled by a valve. Here again, foam production is exclusively effected by means of gas fed in the required amount.
With polyurethane foams, foam formation is not effected by added gases or propellants but by carbon dioxide as a propellant gas which is formed from the isocyanate and water, where, incidentally, polyaddition of isocyanate and polyol occurs. It has been common so far to vary the cell number by varying the pressure of isocyanate injection into the mixing chamber and/or the mixing chamber pressure itself. In order to obtain fine-pored foams, the injection pressure is increased and the mixing chamber pressure is decreased. In order to obtain coarse cells, the isocyanate injection pressure is decreased and the mixing chamber pressure is increased. However, such measures are limited particularly due to sealing problems occurring at the stirrer shaft. Furthermore, these adjustments are non-reproducible and must be re-optimized frequently.
Another measure for producing more coarse-cell foams of polyurethane consists in degassing the isocyanate employed. This is effected, for example, by prolonged storage in a storage tank, preferably at reduced pressure. It is a measure which requires a lot of time and space and thus is correspondingly costly. The amount of gas dissolved in the isocyanate is very low relative to the carbon dioxide formed but apparently acts as a nucleator for gas bubbles and thus has an effect on the cell size of the foams. Immediately after production, the isocyanate contains little or no dissolved gases but depending on time, absorbs indefinite amounts of air by dissolution on storage and transport. Thus, in the user's process tank, there are irregular layers of isocyanate with varying concentrations of air, possibly resulting in unpredictable variations in cell number during foam production.